89 lines
2.2 KiB
C
89 lines
2.2 KiB
C
/* origin: FreeBSD /usr/src/lib/msun/src/e_logf.c */
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/*
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* Conversion to float by Ian Lance Taylor, Cygnus Support, ian@cygnus.com.
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*/
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/*
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* ====================================================
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* Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
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*
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* Developed at SunPro, a Sun Microsystems, Inc. business.
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* Permission to use, copy, modify, and distribute this
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* software is freely granted, provided that this notice
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* is preserved.
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* ====================================================
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*/
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#include "libm.h"
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static const float
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ln2_hi = 6.9313812256e-01, /* 0x3f317180 */
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ln2_lo = 9.0580006145e-06, /* 0x3717f7d1 */
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two25 = 3.355443200e+07, /* 0x4c000000 */
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/* |(log(1+s)-log(1-s))/s - Lg(s)| < 2**-34.24 (~[-4.95e-11, 4.97e-11]). */
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Lg1 = 0xaaaaaa.0p-24, /* 0.66666662693 */
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Lg2 = 0xccce13.0p-25, /* 0.40000972152 */
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Lg3 = 0x91e9ee.0p-25, /* 0.28498786688 */
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Lg4 = 0xf89e26.0p-26; /* 0.24279078841 */
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static const float zero = 0.0;
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float logf(float x)
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{
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float hfsq,f,s,z,R,w,t1,t2,dk;
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int32_t k,ix,i,j;
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GET_FLOAT_WORD(ix, x);
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k = 0;
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if (ix < 0x00800000) { /* x < 2**-126 */
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if ((ix & 0x7fffffff) == 0)
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return -two25/zero; /* log(+-0)=-inf */
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if (ix < 0)
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return (x-x)/zero; /* log(-#) = NaN */
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/* subnormal number, scale up x */
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k -= 25;
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x *= two25;
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GET_FLOAT_WORD(ix, x);
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}
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if (ix >= 0x7f800000)
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return x+x;
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k += (ix>>23) - 127;
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ix &= 0x007fffff;
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i = (ix + (0x95f64<<3)) & 0x800000;
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SET_FLOAT_WORD(x, ix|(i^0x3f800000)); /* normalize x or x/2 */
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k += i>>23;
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f = x - 1.0f;
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if ((0x007fffff & (0x8000 + ix)) < 0xc000) { /* -2**-9 <= f < 2**-9 */
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if (f == zero) {
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if (k == 0)
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return zero;
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dk = (float)k;
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return dk*ln2_hi + dk*ln2_lo;
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}
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R = f*f*(0.5f - 0.33333333333333333f*f);
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if (k == 0)
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return f-R;
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dk = (float)k;
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return dk*ln2_hi - ((R-dk*ln2_lo)-f);
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}
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s = f/(2.0f + f);
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dk = (float)k;
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z = s*s;
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i = ix-(0x6147a<<3);
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w = z*z;
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j = (0x6b851<<3)-ix;
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t1= w*(Lg2+w*Lg4);
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t2= z*(Lg1+w*Lg3);
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i |= j;
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R = t2 + t1;
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if (i > 0) {
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hfsq = 0.5f * f * f;
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if (k == 0)
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return f - (hfsq-s*(hfsq+R));
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return dk*ln2_hi - ((hfsq-(s*(hfsq+R)+dk*ln2_lo))-f);
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} else {
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if (k == 0)
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return f - s*(f-R);
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return dk*ln2_hi - ((s*(f-R)-dk*ln2_lo)-f);
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}
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}
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